This invention relates to systems for detecting the presence of magnetic particles in fluids, and is more particularly but not exclusively concerned with such a system for use in an engine, typically a gas turbine engine, to detect the presence of magnetic particles in the oil used to lubricate the engine.
The relatively movable contact surfaces (eg in bearings, on shafts) of many engines are often made from magnetic materials, typically iron and/or nickel alloys. The quantity and size of magnetic particles in the oil used to lubricate such an engine are therefore frequently indicative of engine wear, since the particles typically originate by being worn or rubbed off the contact surfaces by friction. A sudden increase in the quantity and/or size of the particles can be an indication of rapidly increasing wear, or even incipient failure of a component of the engine, for example a bearing. It would therefore be highly desirable to be able to detect the presence of such magnetic particles "on line", ie while the engine is operating, so that if particles indicative of an incipient failure are detected, the engine could be shut down.
Various optical systems are known for detecting the presence of particles in fluid, eg by measuring the scattering of light by the particles. However, engine oil usually contains, in addition to magnetic particles, particles of carbon and other combustion products produced in the operation of the engine, and the known optical systems would not be capable of reliably distinguishing between the magnetic particles and those other particles.
It is therefore an object of the present invention to provide a system which can detect the presence of magnetic particles in a fluid, while ignoring the presence of non-magnetic particles.